Not Applicable.
The present invention relates to air-depolarized power sources having an electrode such as metal-air battery cells and, in particular, relates to an improved method and apparatus for providing an adequate supply of oxygen to air-depolarized cells.
The vast majority of portable electronic devices can be battery operated. The battery or batteries required to operate such devices are typically inserted into a cavity within the device or are attached to an external surface of the device. Of greatest interest in the marketplace today are so-called high current drain portable consumer electronic devices such as cell phones, digital cameras, flash cameras, computers, personal digital assistants, cassette players and compact disc players. In many instances, such devices accept alkaline batteries. However, alkaline batteries are not necessarily efficient energy sources for such devices since the energy available from alkaline batteries decreases as the rate of current drain increases. It thus became advantageous to provide an alternative energy source for such devices. The ability to do so was constrained by the existing cavity or surface configurations which are typically sized for a pre-determined number of cylindrical alkaline cells.
Metal-air battery cells were introduced as an improved alternative to alkaline cells for use in a portable electronic device while providing an energy source more appropriate to the high current drain conditions associated with such devices. FIG. 1 depicts the available energy of AA premium alkaline and AA zinc-air cells at various power draws. It is apparent from FIG. 1 that the energy available in the typical power ranges of 100 mW to 1000 mW is much greater in zinc-air cells than in alkaline cells. Accordingly, it is desirable to substitute zinc-air cells in place of standard alkaline cells. For example, a pair of AA alkaline cells in a digital camera have an expected operating life of less than xc2xd hour. In contrast, zinc-air cells in a digital camera can provide several hours of operation, and can be readily be replaced by the user when discharged.
Metal-air cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. For example, in a zinc-air battery, the anode contains zinc, and during discharge, oxygen from the ambient air and water from the electrolyte is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode of a metal-air battery utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. Metal-air battery cells are often arranged in multiple cell battery packs within a common housing to provide a sufficient amount of power output. The result is a relatively lightweight battery.
Both primary and secondary metal-air batteries have been developed. A rechargeable metal-air battery is recharged by applying voltage between the anode and cathode of the metal-air battery cell and reversing the electrochemical reaction. Oxygen is discharged into the atmosphere through the air permeable cathode.
One difficulty associated with metal air batteries is that the ambient humidity level can cause the metal-air battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 45%. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will absorb water from the air through the cathode and fail due to a condition called flooding, which may cause the battery to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery is used, failure occurs from drying out.
The problems caused by ambient humidity are exacerbated in air depolarized cells because the oxygen diffusion electrode (cathode) typically passes water vapor as freely as oxygen due to the similar size and polarization of gaseous water molecules. Thus, as air is supplied to such batteries on discharge, or vented on recharge (in the case of rechargeable batteries), water vapor freely passes through the cathode as well.
Therefore, the art has recognized that an imbalance between the humidity level in the air passing over the air cathode and the humidity level within the cell creates a net transfer of water into or out of the cell, and may lead to the problems outlined above. Furthermore, such problems become more serious when large quantities of new ambient air continuously flow over the cathode.
Another problem associated with supplying a metal-air cell with continuous supplies of fresh air is the transfer of carbon dioxide into the cell, which neutralizes the electrolyte such as potassium hydroxide.
In order to make such cells more useful over longer periods of time, air managers have been developed, which isolate the cells from the environment when they are not in use, but provide air when needed. The system disclosed in U.S. Pat. No. 4,913,983 encloses metal air cells in a housing having an air inlet and outlet. A baffle within the battery housing is used to open or close the air inlet and outlet, limiting air access when the cells are not in use. The system also has a fan to supply a greater flow of ambient air to the cells when needed. This arrangement achieves a continuous flow of new ambient air across the air cathodes at a flow rate sufficient to achieve the desired power output. More advanced air managers have been developed which also include a cell enclosure, and an air mover, such as a fan, but use diffusion tubes rather than a closable baffle to isolate the cells from the environment. In particular, the cell receives open air through the diffusion tubes, which limit the amount of fresh air reaching the cell when it is not in use sufficiently to reduce dryout or flooding, and carbon dioxide absorption, which would further reduce the cell capacity. When the cell is in use, the air mover forces air through the diffusion tubes, bringing fresh air through the enclosure.
Examples of such air manager are disclosed in U.S. Pat. Nos. 5,356,729, 5,560,999, and 5,919,582, the disclosure of each of which is hereby incorporated as if set forth in its entirety herein. In the ""729 patent, a housing is typically provided that encloses at least one metal-air cell having at least one ventilation opening that is sized to preferentially diffuse oxygen into the housing upon reduction of the oxygen concentration within the housing caused by operation of the cell or cells. A fan is positioned to circulate and mix gases which are present within the housing. Accordingly, the need of the cell for oxygen is met while maintaining a more stable water vapor and carbon dioxide equilibrium across the air cathode.
The purpose of previous air managers thus has been to isolate the cells from the environment, extending the usable life of the cells. The air manager disclosed in the ""729 patent is designed to be used in either open air or in places (such as battery compartments of some electronic devices) where there would be enough fresh air circulation for a metal-air cell to be used at the desired rate without using an air manager. In particular, the air manager is able to only move air within the air manager""s internal housing, but can not influence the freshness of the air surrounding the air manager because of the relatively low air flow rate. Accordingly, when the metal-air cell is used inside a battery compartment where an insufficient amount of oxygen is present to support the operation of the cell at the desired rate, the air flow described in the ""729 patent would be insufficient. The cell would thus eventually starve for oxygen, thus significantly reducing the energy output during operation.
One solution would be to move a larger quantity of air through the air manager, over the cells, and through the battery compartment. While this would provide enough oxygen for the cells to operate at the desired rate, the cells would dry out prematurely or flood due to the high air flow rate. Furthermore, most electrical devices demand varying amounts of power during operation, depending on the function being performed. If the air manager is configured to supply a constant flow of oxygen to the cathode sufficient to operate the device at its highest power level, the air supply will cause excessive environmental exposure for the zinc air cell causing premature failure when, for example, the device demands only a fraction of the maximum power. Moreover, large power demands are placed on the cell for operating the fan unnecessarily at high speeds.
While conventional air managers are effective in achieving the function of circulating air that is disposed in the battery compartment throughout the battery to stimulate the cell, they are ineffective at providing fresh air to the cell when the battery is installed in a location having restricted access with respect to the ambient environment. Otherwise stated, conventional air managers do not add to the operability of a cell that would be otherwise inoperable when disposed in a battery compartment where fresh air is not abundantly available. This is primarily because conventional air managers are designed to operate in environments having direct access to the fresh air of the ambient environment. For example, traditional cellular phone and camcorder batteries have surfaces in communication with the ambient air where air inlets and outlets are located. As a result, conventional air managers, which produce an air flow between 4 and 6 times more than the flow needed by the cathode for stoichiometric operation of the cells, are able to draw sufficient fresh air in these xe2x80x9copenxe2x80x9d environments to maintain a sufficient oxygen concentration at the cathode. However, research has indicated that a greater flow rate is necessary when the battery compartment is not open, but rather receives air from the ambient environment indirectly (e.g. via relatively small leaks in the compartment). However, as noted above, if conventional air managers are modified to produce a greater flow rate, the life of the battery will be significantly reduced.
What is therefore needed is an air manager for a metal-air cell that enables fresh air to circulate throughout the cell when installed in compartments having a limited oxygen supply, and that supplies only the necessary amount of oxygen to the cells to increase the life span of the battery.
In one aspect the invention provides a battery that is configured to be installed in a battery compartment of an electrical device. The battery has at least one air-depolarized cell to supply power to the device. The battery includes a housing that defines a cell cavity containing the cell. The housing includes a bypass airflow conduit extending through the housing and isolated from the cell, a second conduit defined by a gap between the cell and the cell cavity, and a housing inlet in fluid communication with the bypass airflow conduit and the second conduit. An air manager is provided having an air mover configured to supply air to the inlet. A first portion of the air travels along the bypass airflow conduit to stimulate air flow within the battery compartment, and a second portion of the air travels along the second conduit to deliver oxygen to the cell.
In one preferred form, the air mover receives air from the battery compartment, and outputs the received air into the housing inlet. The first and second portions of air are exhausted from the housing and flow into the battery compartment via a housing outlet. A first portion of the exhaust air exits the battery compartment, and a second part of the exhaust air re-circulates back to the air mover. The air mover may operate at variable speeds to ensure that only the air flow necessary to operate the device flows past the cell.
In another preferred form, the housing is formed from a pair of adjacent cylindrical cell cavities having a xe2x80x9cfigure-8xe2x80x9d cross sectional configuration, such that the bypass airflow conduit is centrally disposed between the cavities. Each cell cavity is sized to receive at least one of a AA, AAA, AAAA, C, and D sized cell.
In one form, the first portion of air and the second portion of air exit together via an outlet diffusion tube that extends between the bypass airflow conduit and the battery compartment.
In another form, the first portion of air exits the bypass airflow conduit into the battery compartment, and the second portion of air exits the second conduit into the battery cavity via an outlet diffusion tube that extends between the second conduit and the battery compartment.
In another form, an inlet diffusion tube extends between the air mover and the second conduit, such that the second portion of air travels from the air mover to the second conduit via the inlet diffusion tube.
Preferably, the air mover is disposed in a chamber that is removably connected to the housing. The air mover may be a standard fan. Alternatively, the air mover could be formed from a flexible tubing that extends between an inlet of the chamber and the housing inlet, a rotatable pump head having protrusions extending therefrom configured to compress and subsequently depress the flexible tubing, and a pump motor operable to rotate the pump head to drive air through the tubing and into the housing inlet.
In accordance with another aspect of the invention, the battery does not include the bypass airflow conduit, but rather has a housing inlet disposed in an axially upstream wall of the housing, and a housing outlet disposed in an axially downstream wall of the housing and aligned with the housing inlet. A conduit is defined by a gap between the cell and the cell cavity, and the air mover of an air manager supplies air to the housing inlet. A first portion of the air travels axially towards the housing outlet to stimulate air flow within the battery compartment, and a second portion of the air travels along the conduit to deliver oxygen to the cell.
In another aspect, methods are provided for using these batteries.
The present invention recognizes that the single function achieved by conventional air managers is inadequate to provide reliable battery operation in locations where fresh air is limited, and overcomes this deficiency by providing a dual-function air manager that 1) draws fresh air into the battery compartment, and 2) circulates a portion of the fresh air over the cell cathode to activate the battery.
The present invention further provides a air-depolarized cell cartridge that is easier and less expensive to manufacture, is self-sealing for reduced leakage, has a thin outer wall to increase power and capacity, and allows a maximum amount of air to flow therethrough and stimulate air flow in the battery compartment.
The foregoing and other aspects of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration, and not limitation, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must therefore be made to the claims herein for interpreting the scope of the invention.